The present invention relates to testing of optical transmission systems, and more particularly to an arrangement and method for producing a predeterminable polarization mode dispersion (PMD).
Such arrangements are required, for example, for producing a certain PMD for measurement or test systems, or for compensating PMD-related distortions in optical transmission systems, and particularly transmission fibers. Since each optical transmission line contains double refracting elements, light signals of different polarizations travel through the line at different group velocities. At a receiver, the light components of different polarizations therefore arrive time-delayed relative to each other. This transit time differential leads to a broadening of the received signal and hence to an impairment of the transmission quality. This can particularly lead to an increase in bit error rate.
Polarization mode dispersion includes all polarization-dependent transit time effects for which the signal propagation may be described fully by the propagation behavior of two polarization modes that are independent of and orthogonal to one another. Since the double refraction may change constantly as a result of external influences such as temperature changes and mechanical load, and also depends on the wavelength, both the position of the principal states of polarization (PSP) and the transit time difference between the PSPs change in the same way. These effects are also referred to as second-order polarization mode dispersion. From the mentioned effects there results a wavelength-dependent PMD behavior fluctuating over time with time constants in a wide time range.
A known arrangement exhibits, in principle, a first polarization splitter/combiner element that splits the incoming signal into two signals having polarization directions which are perpendicular to one another, a delay unit that is arranged in one of the signal lines of both (split) signals, and a second polarization splitter/combiner element that combines the two split signals once again. This known arrangement has the disadvantage that it only allows for the compensation of first-order PMD-related distortions. However, especially in the case of long transmission distances, second-order PMD-related distortions play a significant role.
Another known arrangement for PMD emulation is, for example, an arrangement of a number of double refracting elements, particularly of PM fibers with different or the same group transit times, each of which has a polarization transformation unit installed upstream. This is where the necessary split into the two orthogonal polarization levels occurs as a result of the characteristic of the PM fiber having different propagation velocities for its PSP.
What is desired is an arrangement for producing a predeterminable polarization mode dispersion that also allows the production of a second-order polarization mode dispersion, and which reproduces the polarization mode dispersion of a real transmission fiber as exactly as possible.
Accordingly the present invention provides an arrangement for producing a predeterminable polarization mode dispersion having a first double refracting element or polarization splitter/combiner element rotatable around the direction of propagation of the light that splits the input signal into two signals with polarization directions perpendicular to each other. The first double refracting element passes on the orthogonal polarizations with transit times that are different relative to each other. A second double refracting element or polarization splitter/combiner element also is rotatable around the direction of propagation of the light on which the orthogonally polarized input signals exit as a combined output. There is a rotating element that twists the polarization principal axes behind the second double refracting element by an appropriate angle relative to each other. The light signal exiting the rotating element is fed into an arrangement similar to that before the rotating element. The angle by which the rotating element twists the polarization principal axes relative to each other is adjustable. This makes it possible to adjust the steepness of the principal state of polarization (PSP) and the ratio of the first and second-order polarization mode dispersions (PMD) that make up the predeteminable polarization mode dispersion.